Tectonic Backgrounds,diagenesis And Mineralization Of The Precambrian Magma In The Middle Of The Western Yangtze Block

To understand the South China Craton(SCC),which is one of the three cratons in China,the Western Yangtze Block(WYB)is an important area wherein the widespread Mesoproterozoic to Neoproterozoic rocks are the key to find some scientific issues,such as the Precambrian tectonic-magmatic evolution of the Yangtze Block and the SCC.Moreover,as a result of the assembly and break-up of the supercontinent Rodinia,variety of metallogenesis occurred in the WYB,including the Chahe Sn-W deposit in Huili County and the Lugu Fe-Sn deposit in Mianning County,both of which were considered to be related to the Precambrian magmatism in the region.The Precambrian geological issue of the WYB has been debated hotly by international geologists for a long time due to the composite and evolution of the basement of the WYB,as well as the related metallogenesis still need to be answered.In this dissertation,zircon U-Pb ages,petrographic,whole-rock geochemical and SmNd isotopic data are reported from selected fresh samples in the middle of the WYB,based on analysed previous works and published-papers and conducted a lot of field works,in order to constrain their emplacement age,magma source and petrogenesis,as well as their tectonic setting and regional tectonic-magmatic evolutionary process.In addition,the regional metallogeny,as well as the metallogentic geological backgrounds,the ore-controlling factors,and the prospecting indictors of the typical deposits are studied and analysed.Finally,this dissertation interprets the tectonic backgounds,diagenesis and mineralization of the Precambrian magma and targets the related mineral prospects in the middle of the WYB.The main points of the dissertation are listed below:The systemic zircon U-Pb geochronology is studied on the Precambrian igneous rocks in the middle of the WYB,and concludes that there are three Precambrian magmatic events in the region: the late-Mesoproterozoic(1055 Ma~1006 Ma),earlyNeoproterozoic(842 Ma~772 Ma)and mid-Neoproterozoic(750 Ma~728 Ma)magmatic events,respectively.The Mosuoyi granite which was considered as a Neoproterozoic ganite was indentified as a Mesoproterozoic granite,and there are some Neoproterozoic rocks in the Kangding group was also testified.The geochemical and Sm-Nd isotopic characteristics imply that the late Mesoproterozoic Mosuoyi granites were resulted from the convergence of two continental plates and derived from the re-melting of the upper crustal mudstone and/or clastics,and the mid-Neoproterozoic Xilong gabbros were formed in the continental arc and resulted from mantle-derived magma mixed crust material,as mantle-derived magma passed through preexisting crust that contained Precambrian basement.Therefor,the dissertation indicates that the collision between the Yangtze and Cathaysia blocks occurred at about 1055 Ma,consistent with the worldwide Grenvillian orogenic event.The mid-Neoproterozoic A-type granite intruded into the Neoproterozoic metavolcanic-sedimentary unit(the Suxiong group),which was previously considered as S type granite,is identified in Shimian County,Sichuan province.The dissertation concludes that the A-type granite was derived from the ridge subduction and proposes a new geodynamic model,ridge subduction and slab window,to interpret the Neoproterozoic magmaism in the western Yangtze Block,based on the geological background,the types of igneous rocks and its spatial-temporal correlations,the geochemical characters of granites systemically studied,and comprehensively thoughtfully observed.In the study area,the Mosuoying and Lugu granitic intrusions closely related to the Chahe Sn-W deposit in Huili County,and the Lugu Fe-Sn deposit in Mianning County,respectively.Our study newly reveals the emplacement ages,types of granitoids,as well as the tectonic settings of the intrusions and based on that,the dissertation indicates that the Chahe Sn-W deposit and the Lugu Fe-Sn deposit are formed from the different diagenetic and ore-forming dynamic backgrounds in different geological times: The Chahe Sn-W deposit belongs to the mesothermal to hyperthermal sulfide deposit,while the Lugu Fe-Sn deposit belongs to the skarn type.Based on the new model of the diagenetic and ore-forming dynamics of the Precambrian intermediate-acid magam in the middle of the WYB,and according to the principles and Chinese standard of the division of metallogenic belts in China,this dissertation divides the granitic intrusions related Sn-W-Fe metallogenic belt in the WYB as two ore forming units.Moreover,the metallogentic geological backgrounds,ore-controlling factors,prospecting indictors of the typical deposits are studied and analysed.Finally,two metallogenic prospect areas,the Sn-W Metallogenic Prospective Area and the Fe-Sn Metallogenic Prospective Area,are targeted.